Today, there exist various examples of semi-submersible offshore platforms, for receiving modules, such as modules comprising of for example process equipment, electrical equipment or utility equipment.
Semi-submersible offshore platforms do not rest on the seabed. Instead, the working deck sits atop large pontoons and hollow columns, these float high in the water when the semi-submersible offshore platform is moved. At a field location, personnel pumps seawater into the pontoons and columns to partially submerge the semi-submersible offshore platform, hence the name semi-submersible. With much of its bulk below the water's surface, the semi-submersible offshore platform becomes a stable platform for drilling and production, moving only slightly with wind, wave and currents. Semi-submersible offshore platforms are used for various services such as production of hydrocarbons, drilling and/or to provide living accommodation for personnel.
In conventional semi-submersible offshore platforms, a load-supporting, rectangular deck box structure is positioned upon the top of the columns. Modules are then placed on top of the deck box structure. The deck box structure offers a structurally solid design and may be of a sealed type, which adds reserve buoyancy to the semi-submersible offshore platform in an eventual damaged emergency state.
A disadvantage with these known conventional design is that the modules have to be placed relatively high on the semi-submersible offshore platform which leads to a high centre of gravity for the modules, and accordingly for the completed offshore platform. This results in a reduction in stability for the semi-submersible offshore platform and as a consequence a lesser pay-load, or alternatively the size of the semi-submersible offshore platform has to be increased to compensate for the high vertical centre of gravity of the modules. These conventional designs are restricted to provide relatively heavy and bulky semi-submersible offshore platforms. Further, these types of semi-submersible offshore platforms have a limited ventilation capacity of the deck box and may also have difficulties to utilize the deck box space in an effective way.
It is preferred to provide semi-submersible offshore platforms which can lower the centre of gravity for the modules and the semi-submersible offshore platforms. US2005217554 discloses a semi-submersible offshore vessel having an upper deck structure which is generally C-shaped which enables a lower centre of gravity for the modules and the semi-submersible offshore vessel. However, this solution leads to a relatively heavy, bulky and complex semi-submersible offshore vessel. This known example provide limited reserve buoyancy to the semi-submersible offshore platform. Further, this solution is restricted to a time consuming assembly, installation, maintenance, and provide limited construction flexibility. Further, this type of semi-submersible offshore platform has a limited ventilation capacity of the platform.
There is thus a need for an improved semi-submersible offshore platform removing the above mentioned disadvantages.